The coexistence of congenital complete heart block and QT prolongation represents a special type of arrhythmia. The electrophysiological and clinical characteristics of this syndrome were studied in eight patients suffering from congenital AV block and QT prolongation. Data from 22 patients suffering from congenital complete heart block only, served as a control. In the study group, the appearance of a torsade de pointes type of ventricular tachycardia could regularly be observed and the tachycardial attack could usually be provoked by ventricular extrastimuli. The corrected QT time was markedly prolonged; on ventricular stimulation, at higher pacing rates the QT interval shortened, but remained significantly higher than in the control group. Syncopal attacks--with the character of polymorphic tachycardia--appeared in each patient of the study group while occurring in only three patients from the control group. Patients were given pacemaker implants (using a higher pacing rate) and long-term administration of beta-receptor blockers. The outcome was favourable; no ventricular tachycardia or syncopal attack was observed in the follow-up period.